A so-called strobe flash illumination apparatus employing a xenon discharge tube is utilized in various fields as a convenient light source of high efficiency. When used as the original illuminating apparatus in a copying machine, the above-mentioned apparatus is advantageous in respects to low heat generation, excellent color rendering properties and others, assuring a high utility. However, in the case of the copying machine in which an original is simultaneously illuminated over the whole surface thereof, it is necessary to illuminate uniformly the original as a whole. Since such uniform illumination cannot be accomplished solely with the flash lamp by itself, it has heretofore been practiced to make use of a reflector or the like. By using the reflector, a greater proportion of light rays emitted from the lamp can be caused to reach the surface of the original, whereby the utilization efficiency of light energy can be enhanced. However, with regards to the uniform information over the whole surface of the original, this arrangement was inadequate.
When used as strobe flash in the photographing, such a structure is generally adopted in which a collecting and diffusing optical element (stepped concave or convex lens such as a Fresnel lens) is provided on the front side of the reflector. In the case of this structure, the light rays emitted directly from the lamp as well as those reflected by the reflector equally undergo diffusing action. Thus, this structure is dissatisfactory as the means for realizing uniform illumination even though it can change the range within which light rays can reach, because the light rays directly emitted by the lamp and those reflected by the reflector are equally collected and diffused.
FIG. 3 is a view showing an example of illuminance distribution over an original surface obtained by illuminating the original surface from below with inclination by means of lamp units 1 each having a collecting and diffusing optical element 2 and disposed below the original 3 at both sides thereof. Curves shown in the figure represent equi-illuminance curves, wherein numerals inserted between the curves indicate the relative values of illuminance in the associated areas on the assumption that illuminance in the vicinity of the lamp 1 is given by 10. It will be seen that illuminance is decreased to 2 at both side ends in FIG. 2.
In view of the aforementioned drawbacks of the hitherto known original illuminating apparatus employing the flash lamps for use in the copying machine and others, it is an object of the present invention to provide a flash illumination apparatus for use in the copying machines and others which is capable of uniformizing to an acceptable extent the illuminance distribution on an original surface illuminated with flash lamps.